I have experiemented a bit...
I have played with the traction control on my Exige S 240 a bit, both for autocross and on the track. Note also that I have a limited slipn differential (LSD).
I find it generally unobtrusive, not like some other cars. You can still notice it if you are paying close attention. I find it unnecessary while autocrossing and the LSD seems to have the desired effect on sharp corner exit. (It has the undesirable increased understeer on corner entry.)
For autocrossing, I have it set at 8% and find I rarely get into it. When I do, I do not get a sense I lost any time. Its effect is smooth and I do not feel I am pressing the throttle for more speed like I might with the NSX or M3.
On the track, I set it depending on the traction available. In the morning, cold tires, damp track, I will set it at 5-6%. Later in the day, with my confidence up and hot tires, it goes back to 9%.
Yep, I am worried enough about hitting something that I remain cautious. I do not turn it off unless I want to play....
Michael